As a development standard of 802.11n which is a widely practiced wireless local area network (LAN) standard, IEEE 802.11ac standard was defined by the institute of electrical and electronics engineers, Inc. (IEEE). Currently, standardization activities of IEEE 802.11 ax have been performed as a successor standard to IEEE 802.11 n/ac. In the current wireless LAN system, interference due to an increase in the number of terminals per area is becoming a big problem, and it is necessary to consider such overcrowded environment in the IEEE 802.11ax standard. On the other hand, in the IEEE 802.11ax standard, not only improvement in peak throughput but also improvement in user throughput is main requirements, unlike the past wireless LAN standard. It is indispensable to introduce a highly efficient simultaneous multiplexing transmission method (access method) in order to improve the user throughput.
In the standards up to IEEE 802.11n, an access method of autonomous distributed control method referred to as carrier sense multiple access with collision avoidance (CSMA/CA) has been adopted as an access method. In the IEEE 802.11ac, space division multiple access (SDMA) with a multi-user multiple-input multiple-output (MU-MIMO) technology was newly added.
In the IEEE 802.11ax standard, backward compatibility for the existing IEEE 802.11 standard is required. This suggests that it is necessary to support the access method based on CSMA/CA even in the IEEE 802.11ax standard. However, in CSMA/CA which requires carrier sense prior to transmission, there is a problem that communication opportunities are greatly reduced due to interference between terminal devices under the overcrowded environment as described above. Therefore, recently, changing the threshold (CCA level, CCA threshold) of clear channel assessment (CCA) by carrier sense has been discussed for the purpose of allowing some interference and improving communication opportunities (see Non-patent Document 1 or the like). Since the terminal device stops communication if interference not less than the CCA level is measured by carrier sense, increasing the CCA level reduces the possibility that the terminal device loses communication opportunities even in overcrowded environment. Increasing the CCA level naturally causes degradation of reception quality due to interference, but communication quality is expected to be maintained by packet capture effect peculiar to packet transmission and adaptive modulation transmission.